Nokia RX-51: Remove function set_muxconf_regs()

This function is not used and was never called.

This board contains '#define CONFIG_SKIP_LOWLEVEL_INIT' because X-Loader
set everything up, including MUX configuration.

Also this MUX configuration is incorrect and does not match hardware.

So remove this dead, unused and broken code.

This change will decrease size of U-Boot binary.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Acked-by: Pavel Machek <pavel@ucw.cz>